Liquid-crystal modulators are well known. They are most prevalently used in displays ranging in size from wrist watches to flat-panel displays on lap top computers. In such displays, the bias applied to the pixel of the multi-element cell, when used in combination with polarizers, determines whether the pixel absorbs or passes light. Since the output is directly viewed, the ratio of the light passed in the transmissive mode to the light passed in the absorptive mode need not be very high. This ratio is referred to as the extinction ratio for a liquid-crystal cell.
Specialized liquid-crystal optical modulators are also known in which a single, well defined beam strikes the modulator and its intensity is modulated according to the electrical bias applied across the liquid-crystal cell. Many applications of optical modulators require a high extinction ratio.
A relatively new application of liquid crystals involves optical switches in a multi-wavelength optical communication. Brackett et al. in "A scalable multiwavelength multihop optical network: a proposal for research on all-optical networks," Journal of Lightwave Technology, vol. 11, no. 5/6, 1993, pp. 736-753 describe an all-optical communication network based on optical fibers, each carrying multiple optical signals of different carrier wavelengths. The all-optical network requires for its most useful applications switching nodes connecting multiple fibers that can switch the different optical signals between three or more fibers or other optical paths according to their wavelength, all the while the signals are maintained in the optical domain, that is, without any electro-optical conversion.
One type of such optical switch is the liquid-crystal switch described by Patel and Silberberg in U.S. Pat. Nos. 5,414,540 and 5,414,541, both incorporated herein by reference, and in "Liquid Crystal and Grating-Based Multiple-Wavelength Cross-Connect Switch," IEEE Photonics Technology Letters, vol. 7, no. 5, May 1995, pp. 514-516. A schematic representation of a 2-wavelength switch based on this technology is illustrated in perpendicularly arranged views in FIGS. 1 and 2. A two-wavelength optical beam 10, assumed in this simple example to be polarized in they-direction, strikes a frequency-dispersive element 12, such as a Bragg grating to produce two optical beams 14, 16 separated according to their wavelengths. A lens 18 may be required to produce the required optical configuration. The two beams 14, 16 strike respective segments 20, 22 of a segmented liquid-crystal modulator 24 after passing through a first polarization-dispersive element 26, such as a calcite crystal or Wollaston prism. The calcite crystal 26 is arranged such that the y-polarization corresponds to the ordinary polarization of the calcite. The utility of the first polarization-dispersive element 26 is not readily apparent in this simple explanation, but its need become more obvious when two input beams are being switched in an add/drop circuit.
Many aspects of the invention are not directly dependent upon the use of a liquid-crystal modulator, but that example will be used here for definiteness. Each segment 20, 22 of the liquid-crystal modulator 24 constitutes a separately controllable liquid-crystal modulator. More details will be given later, but the liquid-crystal cell 24 has been previously used in configurations which typically include two glass plates with a gap between them which is filled with a nematic liquid crystal. In one embodiment, one side of the segmented modulator 24 has a uniform biasing electrode while the other has an array of electrode fingers. In this configuration, states of polarization are use for switching, as discussed in the cited Patel and Silberberg patents. Depending upon whether electrical bias is applied to the respective segment 20, 22 the polarization of the beam 14, 16 striking the segment either is left in its y-polarization or is rotated by 90.degree. to the x-polarization, which is the extraordinary polarization with respect to the two calcite crystals 26, 28.
After the beams 14, 16 have passed through the liquid-crystal modulator 24 with perhaps the polarization state of one or the other of the two wavelength signals being rotated, the beams pass through a second polarization-dispersive element 28. As shown in FIG. 2, the polarization-dispersive element 28 distinguishes the polarization states of the beams 14, 16 and accordingly transmits the ordinarily polarized light into beams 32, 36 (FIG. 2) and transmits the extraordinarily polarized light into beams 34, 38. Following focusing by a second lens 30, a second wavelength-dispersive element 40 recombines the two beams into either first output beam 42 or second output beam 44, the two output beams 42, 44 being of different polarizations. If the beams exiting the second polarization-dispersive element 28 are of different polarizations, one is directed to the first output beam 42 and the other to the second output beam. It is understood that the two segments 20, 22 allow this switching to be performed independently for each wavelength. Thus, the electrical biasing conditions determine onto which output beam 42, 44 each of the two wavelength-differentiated signals 14, 16 are switched.
This explanation is intended only as an example of the type of multi-wavelength optical switching that is provided by liquid-crystal cells. The example will be used to illustrate some problems addressed by the invention. Many other configurations of liquid-crystal switches and modulators are included within the invention.
The above optical switching networks do not depend critically upon the modulator being based upon a liquid crystal. Such a switching network, particularly when applied to multiple input beams and to beams of mixed polarization, depends upon a selective polarization converter that in one state can pass the light with its polarization unchanged and in another state simultaneously converts TE-polarized light to TM-polarized light and vice versa.
A schematic cross-sectional view of a conventional segmented liquid-crystal modulator 20 is shown in FIG. 3. On one transparent glass plate 50 are formed two semi-transparent electrode fingers 52, 54, for example, of indium tin oxide (ITO), which are connected to respective biasing sources. On the other transparent glass plate 56 is formed a semi-transparent planar counter-electrode 58, also of ITO, typically grounded or biased to a fixed potential. Alignment layers 62, 64 of an organic dielectric material are deposited over the electrodes on both glass substrates 50, 56. The alignment layers 62, 64 are buffed in predetermined directions that are perpendicular to each other when the substrates 50, 56 are assembled together. Typically, the buffing direction on the first substrate 50 is along the long direction of the finger electrodes 52, 54. The two glass substrates 50, 56 are then assembled into a liquid-crystal cell with the buffing directions perpendicular between them and with a gap 66 of thickness d between the two alignment layers 62, 64.
A nematic liquid crystal 68 is then filled into the gap 66. Because of the perpendicularly buffed alignment layers 62, 64, the director of the liquid crystal (i.e., the direction of the long axis of the molecules constituting the liquid-crystal 68) is fixed at the surfaces of the respective alignment layers 62, 64 to lie along the respective buffing directions. In the absence of other forces, the director smoothly varies between the two alignment layers 62, 64. That is, its vector head follows a helix, and the liquid-crystal molecules resemble a 90.degree. screw between the two alignment layers.
Nematic cells for optical displays should satisfy the Mauguin condition, which for a 90.degree. twist is stated as ##EQU1##
where .DELTA.n is the difference in refractive index between the two principal directions of the liquid crystal molecule, d is the thickness of the liquid crystal in the cell, and .lambda. is the wavelength of the light. If a beam of light of light traverses such a gap 66 filled with a twisted liquid-crystal structure and if the light's polarization is parallel or perpendicular to the alignment direction of the incident side, and if the pitch of the helix is sufficiently long to satisfy the Mauguin condition, the helically wound liquid crystal will waveguide the light. As a result, the polarization of the traversing light beam is twisted substantially by 90.degree. upon traversing the liquid-crystal cell in this state.
However, if the electrodes 52, 54, 58 impose an electric field of sufficiently high magnitude across the liquid crystal 68, the liquid-crystal director is forced to be parallel to the electric field which exists across the gap 66 except in areas immediately adjacent to the alignment layers 62, 64. Thereby, the electric field destroys the waveguiding, and the light exits the cell 20 with the same polarization with which it entered. By the appropriate placement of polarizers and analyzers relative to the alignment directions, the voltage applied across the liquid-crystal will change the light characteristic of the cell transmissivity between blocking and transmissive.
Since the twist of the director between the two alignment layers 62, 64 could be either +90.degree. or -90.degree., a chiral dopant is typically added to the liquid crystal 68 to break the symmetry by inducing the twist only in one helical direction, and to thereby avoid scattering from different domains. This solution is well known in the prior art.
For most display applications, extinction ratios of 100:1 (20 dB) or even 10:1 (10 dB) are acceptable for adequate viewing quality. However, the liquid-crystal multiwavelength optical switch of FIGS. 1 and 2 and other such switches present much more stringent requirements. In view of the fact that the output wavelength-dispersive element 40 passes any remnants of a blocked channel onto the output beams 42, 44, a finite extinction ratio is equated with cross talk between channels. For a practical all-optical networks, cross talk introduced by the switching elements needs to be kept as low as possible. For example, if there are two input beams each having the same wavelength comb of signals, a finite extinction ratio means that an output path will carry both the transmitted signal at a particular wavelength switched to that output path as well as residual amounts of the blocked signal at that same wavelength which was principally switched to another output path.
A principal cause for finite extinction ratios in liquid-crystal cells is that the Mauguin condition of Equation (1) is only approximately satisfied in most practical liquid-crystal cells. Scheffer et al. give a more complete expression for the transmissivity T of light through parallel polarizers sandwiching a 90.degree. twisted nematic liquid crystal in "Twisted Nematic and Supertwisted Nematic Mode LCDs," Liquid Crystals: Applications and Uses, vol. 1, ed. Bahadur (World Scientific, 1990), pp. 234-236, specifically, ##EQU2##
where ##EQU3##
with the previously defined quantities. The transmissivity T thus depends upon the thickness d with the dependence defined in Equation (2). Although the transmissivity T is relatively small for values of u greater than 1, it assumes a zero (minimum) value only for a discrete set dependent upon the positive even integers EQU 1+u.sup.2 +L =2, 4, 6, . . . , (4)
which can be alternately expressed as EQU u=1.732, 3.873, 5.916, . . . (5)
The values stated in either Equation (4) or (5) are known as the first, second, and third minimum conditions respectively and represent conditions for which exact polarization conversion occurs.
Thus, only for discrete values of cell thickness d does the extinction coefficient assume an infinite value. For laboratory purposes, the liquid-crystal cells can be customized and the optical setup temporarily optimized to achieve nearly ideal characteristics. However, as the liquid-crystal optical switches move out of the laboratory into the field, such stringent cross-talk requirements are becoming very difficult to achieve with the conventional liquid-crystal cell. Cells used in verifying the invention have typical lateral dimensions of about 1/2 inch (1 cm) and maintaining gaps of a few micrometers, as required for complete matching of the gap to the minimum condition of Equation (4) or (5) over these dimensions has generally been infeasible with reasonably priced components and simple fabrication techniques.